1. Field of the Invention
This invention pertains to an overvoltage protector element for an electrical circuit. More particularly, this invention pertains to a fail-safe device for such a protector element.
2. Description of the Prior Art
In the telecommunications industry, the use of overvoltage and overload protectors is common to prevent damage or injury resulting from overvoltage conditions in a telecommunication line. Prior art for overvoltage protector elements include gas tube protector elements which have cylindrical bodies with a tip lead, a ring lead and a ground lead extending generally parallel from the body. During an overvoltage condition, the protector element grounds the tip and ring lead to the ground lead. A characteristic feature of such prior art overvoltage protector elements is that they attain an elevated temperature during periods of prolonged grounding
A short period of grounding may result from a brief overvoltage condition. This could occur from lightening strikes. Prolonged grounding may result from prolonged overvoltage conditions. For example, a high voltage power line may fall and rest on a telephone line resulting in a prolonged overvoltage condition on the telephone line.
Any grounding through the gas tube will increase the temperature of the tube. Prolonged grounding resulting from prolonged overvoltage conditions will result in significant temperature increases. The increased temperature presents fire and other risks including personal injury exposure. To minimize these risks, prior art overvoltage protectors utilized so-called fail-safe devices. Such devices would by-pass the grounding of the gas tube and directly ground the tip or ring leads to the ground lead. As a result, extreme increases in the gas tube temperature would be avoided.
Prior fail-safe devices have utilized the elevated temperature characteristics of a failed overvoltage protector element. (Within this application, a failed overvoltage protector element means any element which is experiencing elevated temperature resulting from prolonged overvoltage conditions. The term does not necessarily mean a faulty element.) Specifically, such fail-safe devices used a spring contact having spring ends opposing contact surfaces which were connected to the tip and ring leads. The contact ends of the spring would be spaced from the contact surfaces by a dielectric spacer (for example a plastic disk). The dielectric spacer was made of a material which was deformable upon heating of the material to the elevated temperature. The spring would be connected to the ground lead. Upon failure of the overvoltage protector element, the dielectric material would melt and the spring ends would contact the contact surface resulting in a ground.
Overvoltage protector elements and their fail-safe devices are mass produced. Accordingly, it is desirable to find a design of such a product which can be manufactured and assembled at low cost. Furthermore, it is desirable to provide for design of such a product which has enhanced reliability in the event of failure of the overvoltage protector element to provide a ground.
Also, prior art fail-safe devices result in significantly enhanced operating temperatures. For example, the spring contact of the prior art fail-safe device was stainless steel. Such material is of relatively low conductivity. Also, the dielectric spacers were placed between the gas tube and the intended point of contact of the spring. During fail-safe operation, a thin film of melted dielectric could exist between the spring and the tube. This would lead to increased electrical resistance and resulting increased operating temperature.